cc: Get BeginFrameArgs for STP::BeginMainFrame while inside an impl frame.

cc: Get BeginFrameArgs for BeginMainFrame while inside an impl frame.

This patch also adds asserts to make sure that the assumption that
ScheduledActionSendBeginMainFrame is only called inside an impl frame
holds.

Sending a BeginMainFrame requires BeginFrameArgs to be read from the
LTHI which are only valid inside an impl frame. Due to needing to call
BeginMainFrame asynchronously (as the comment inside the function
mentions), the call could theoretically occur outside an impl frame.
The likelihood of this effecting production is extremely low because
impl frames are large but this problem does increase flakiness of the
layer tree host unittests.

BUG=346230
R=brianderson,enne

Committed: https://crrev.com/69fd3bb5a6824436bb7c3f5905dc3ab3051b9d3b
Cr-Commit-Position: refs/heads/master@{#327874}

[mithro-old, 2015-04-29 01:13:32]

Hi Brian / Enne,

This is another patch related to trying to test
https://codereview.chromium.org/1099703004/. 

At Enne's suggestion I looked at adding asserts to check that made sure the
BeginMainFrame was only sent inside the BeginImplFrame. This lead me to discover
this issue with the the scheduler path inside the SingleThreadProxy.

I wasn't able to come up with a test which demonstrates the issue which also
didn't require the landing of LTHI::Now() fixes. I'll upload the test I came up
with in another CL that I plan to land after the LTHI::Now() fix.

Thanks for your review,

Tim 'mithro' Ansell

[sunnyps, 2015-05-01 00:58:10]

sunnyps@chromium.org changed reviewers:
+ sunnyps@chromium.org

[sunnyps, 2015-05-01 00:58:10]

LGTM

[mithro-old, 2015-05-01 02:54:06]

The CQ bit was checked by mithro@mithis.com

[commit-bot: I haz the power, 2015-05-01 02:54:28]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1110183004/20001

[commit-bot: I haz the power, 2015-05-01 03:45:11]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-05-01 03:46:04]

Patchset 2 (id:??) landed as
https://crrev.com/69fd3bb5a6824436bb7c3f5905dc3ab3051b9d3b
Cr-Commit-Position: refs/heads/master@{#327874}